Typically the compounding of thermoplastic polymeric materials, or admixing of the composition ingredients, is carried out with very costly and massive apparatus, such as a Banbury or similar large volume internal mixer, or an open roll mill. The means and procedures therefor are not unlike those applied in compounding elastomers or rubber generally. In such conventional compounding procedures, large masses of the polymeric or elastomeric material, for example, 300 to 500 pound chunks, are heat fluxed and masticated to reduce the tenacious characteristic of the material to a workable plastic consistency whereupon the other compound ingredients are added and worked through the resulting plastic body with intensive kneading type mixing action.
However, when working with these large batches in typical production apparatus of this type, the overall operation and its control becomes so unwieldy that many commercial manufacturing operations require a combination of two or more types of mixers, such as a Banbury operating in sequence with an open roll mill and followed by a pelletizer, whereby the compounding is carried out in a series of separate means and steps.
Also, the complexity of performing common compounding operations with such apparatus of limited flexibility, creates high manpower requirements as well as rather exacting demands upon the performance of operating personnel. For instance, the intractable consistency of a typical polymeric material, and in turn, its receptivity to blending with the other ingredients is largely dependent upon the internal temperatures and heat control throughout its mass. Moreover, its receptivity to blending may also be influenced to varying degrees by the type and amount of added ingredients, among other possible variable factors. Thus, when handling such large masses of polymeric material as in most commercial operations, internal temperatures throughout a voluminous mass and the dispersion of the added ingredients therethrough without incurring adverse effects, requires a deft balancing of the compound processing conditions. Overmixing tends to degrade polymeric compositions due to excessive mechanical action disruption and/or shearing of the molecular structure of the polymer, and/or over heating of the polymeric material or ingredients contained therewith resulting from the additional heat generated internally from the continued mixing action. Thus, the compounding operation should be gauged so as to terminate the mixing as soon as an adequate degree of dispersion of components through the polymeric mass has been attained.
Moreover, the kneading type of mixing action provided by a Banbury internal mixer or roll mill is not particularly effective in disrupting small agglomerates of finely divided filler ingredients which are commonly encountered due to storage, handling, etc., whereby agglomerates of filler particles are likely to be dispersed and blended with the polymeric material rather than substantially individual particles of filler.
Other mixing systems or apparatus have apparently been tried by the art in compounding thermoplastic materials in an effort to avoid the handicaps of the foregoing common prior art approach, but their application has not been found to provide sufficiently effective and practical blending or cost reductions to replace the prior longstanding systems.
For example, U.S. Pat. No. 3,372,139, makes reference to the prior art methods of utilizing "customary mixing apparatus, such as roller mills and mixing kneaders", and then discusses the preparation of curable polyolefin compositions by a different means whereby improved distribution of the curing agent for the polymer is the primary objective. This patent discloses a method of heating a polyolefin material, preferably in powdered form, together with all other ingredients except the curing agent, up to a "sinter point" in a rapidly rotating eddy mixer, then terminating the heating of the mixture and cooling it whereupon the curing agent is introduced with continued mixing. The "sinter point" is described in this patent as "the vicinity of the softening point of the corresponding polymer" which appears consistent with the standard dictionary definition of becoming coherent by heating without melting. Aside from imposing a significant operational change of reversing from heating to cooling at a stage conditioned upon variable softening temperature ranges for different thermoplastics, with the accompanying responsibility therefor upon operating personnel, it appears that in the method of this patent the thermoplastic powder is softened or sintered whereupon it integrates into a coherent plastic mass not unlike that produced with the more common prior art mixing apparatus such as a Banbury. Morever, the preferred powdered form for the polymeric material called for in the patent is the most expensive form for such materials which adds to cost.
Other prior art disclosures of the use of rapid or high-speed rotating blade mixers, such as the type shown in U.S. Pat. No. 2,945,634, report different effects upon thermoplastic particles and additives. For example, U.S. Pat. No. 3,484,262, as well as an article published in the Wire Journal, "Improve Your Dry Blending of Wire Insulation PVC" by G. L. Levy, August, 1971, PP 39 to 43, disclose that such high-speed mixing apparatus or the conditions imposed thereby, referred to as "abrasive adhesion method" in the patent, produce a product wherein the additives such as pigments or fillers become forcibly driven into and physically embedded in the surface of the polymer particles. For instance, U.S. Pat. No. 3,484,262 states that the "particles of polymeric material have pigment tightly adhered to its surface portions only, the adherence appearing to involve the entrapment of pigment within surface depressions or indentures in the outer regions of the particle . . . ", and the author of the article concludes that the high-speed intensive mixing "causes the filler particles to adhere strongly to the surface" of the polymer particles, undoubtedly made more receptive by the softening effect of the plasticizer which was included.
A different approach to the mixing of thermoplastic materials with solid fillers is proposed in U.S. Pat. Nos. 2,445,928 and 2,572,068 wherein the thermoplastic material is melted and a fine spray of the liquid is applied to the solid filler while the solid filler particles are suspended in air by means of strong agitation with an impeller. The exposure of the suspended filler particles to collision contacts with the liquified plastic is said to cause adherence to each other by impact. This process however requires a separate melting system and spray means, among other rather involved equipment components disclosed in these patents.